Vapor-phase dimers, hydrogen bonding

In addition to frequency shifts, new vibrational bands occasionally appear in the liquid state as compared to the gas phase. An example is formic acid, HCOOH. At high temperatures, the vapor shows a characteristic O—H stretching vibrational band at 3750 cm-1 as the temperature of the gas is lowered, this band disappears and a new band at 3080 cm -1 appears. In the liquid state, only the 3080 cm"1 band is evident. The new band is due to O—H vibration in the hydrogen-bonded dimer. Hydrogen bonding also affects the IR spectra of liquid CH3OH and H20. 

Table 12. Comparison of intermoleeular distances and hydrogen-bond energies in vapor-phase dimers and crystals...

In the vapor phase formic acid forms a hydrogen-bonded dimer ... 

FIG. 3 The vapor phase water dimer structure. Polar covalent bonds are shown as solid lines and the hydrogen bond as a dashed line (adapted from Ludwig, 2001). 

Dehydrodimerization. On excitation with a mercury vapor lamp, mercury is converted to an excited state, Hg, which can convert a C—H bond into a carbon radical and a hydrogen atom. This process can result in dehydrodimerization, which has been known for some time, but which has not been synthetically useful because of low yields when carried out in solution. Brown and Crabtree1 have shown that this reaction can be synthetically useful when carried out in the vapor phase, in which the reaction is much faster than in a liquid phase, and in which very high selectivities are attainable. Secondary C—H bonds are cleaved more readily than primary ones, and tertiary C—H bonds are cleaved the most readily. Isobutane is dimerized exclusively to 2,2,3,3-tetramethylbutane. This dehydrodimerization is also applicable to alcohols, ethers, and silanes. Cross-dehydrodimerization is also possible, and is a useful synthetic reaction. 

The assumption of a strict, vapor-phase derived pair potential appears acceptable only in those cases where a weak intermolecular interaction does not cause appreciable structural relaxations in the monomers. In the case of hydrogen-bonded systems, the use of the frozen monomer assumption precludes, however, almost always the investigation of all the observable structural and spectroscopic features of the A—H moiety. Therefore, the reference system for the discussion of cooperative, nonadditive effects is exclusively the structurally fully optimized hydrogen-bonded dimer with a single isolated hydrogen bond and with all the properties derivable from the global 3N-6 dimensional potential energy surface of the dimer. 

More complicated catemer patterns are frequently encountered in organic crystals. Particularly interesting examples are the first two carbonic acids, formic acid and acetic acid. Both, as with most other carbonic acids, form doubly hydrogen-bonded, cyclic dimers in the vapor phase, but chains with one hydrogen bond linking the monomers in the molecular crystal [34-40]. The structures are sketched in Figure 8.11. Propionic acid [316] and others... 

The simplest example of self-association in the vapor phase is dimerization, as exemplified by hydrogen bonding in caiboxylic acids. Consider the dimerization of acetic acid ... 

Formic acid is miscible with water and most polar organic solvents, and somewhat soluble in hydrocarbons. In hydrocarbons and in the vapor phase, it consists of hydrogen-bonded dimers rather than individual molecules. Owing to its tendency to hydrogen-bond, gaseous formic acid does not obey the ideal gas law. Solid formic acid (two polymorphs) consists of an effectively endless network of hydrogen-bonded formic acid molecules. This relatively complicated compound also forms a low-boiling azeotrope with water (22.4%) and liquid formic acid also tends to supercool. 

Although all these features do occur in open chain hydrogen bonded dimers formed between neutral molecules in the vapor phase, they are mostly present to a minor extent only. Exceptions are complexes between strong acids and strong bases. If one assiomes,... 

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